Process control plants and systems use control valves in a wide variety of applications. Typically, control valves are used to manage product flow by functioning like a variable orifice or passage. A control valve is generally automated using an actuator and a remotely operated instrument which communicates between a process control computer and the actuator to command flow changes within the valve to achieve the desired control strategy. Travel sensors that measure the position of the valve play a critical role in maintaining accurate process control.
When the process control computer issues a command to modify flow, the remotely operated instrument must read the present valve position and apply appropriate corrective action through the actuator. A typical actuator is driven by a pressurized air source, which is controlled by the remotely operated instrument. For example, in a spring and diaphragm actuator used on a linear or sliding stem valve, variations in air pressure applied to a large diaphragm cause movement or displacement of the diaphragm. Attached to the diaphragm is an actuator stem, which in turn is connected to a valve plug. By changing air pressure to the diaphragm, the remotely operated instrument can directly position the valve plug and therefore control flow through the control valve. In order to properly control flow, the instrument must always know where the valve plug is and where it must move to in response to a command. This is accomplished by attaching a travel sensor between the remotely operated instrument and the actuator stem. The output of the travel sensor may be directly connected to the remotely operated instrument to provide stem position feedback for precise valve control.
Traditional travel sensors, such as potentiometers, require moving mechanica linkages to couple movement or displacement into the sensor. However, mechanical linkages have contact or wear points. As such, during rugged service conditions, instabilities can break the mechanical linkages at the wear points thereby disconnecting the valve stem from the remotely operated instrument. To improve sensor reliability, recent sensor designs have migrated to non-contacting position detection methods.
One type of non-contacting sensor design is a magnetic travel sensor, which detects displacement between two objects by attaching a feedback element (e.g., a magnetic flux source) to the first object and a sensing element (e.g., a magnetic sensor) to the second object. The feedback element produces a magnetic field that is detected by the sensing element. Any movement by one or both objects producing relative displacement presents a different portion of the magnetic field to the sensing element, thereby changing the output of the sensor. This output can be directly related to the relative displacement between the actuator and the valve stem.
The accurate functioning of the magnetic travel sensor depends on the proper alignment of the feedback and sensing elements. However, due to factors such as improper installations, excessive vibrations, and/or other environmental effects, the feedback element may become loose and move over time. This can result in misalignment with respect to the sensing element which in turn can cause the sensor to become inoperable.